(a) Field of the Invention
The present invention relates to a method for increasing the molecular weight of polypropylene having a syndiotactic structure. More specifically, it relates to a method for increasing the molecular weight of polypropylene having a substantially syndiotactic structure by partially crosslinking the polypropylene.
(b) Description of the Prior Art
Isotactic polypropylene is utilized in various applications, because of being inexpensive and because of having the relatively good balance of physical properties. On the other hand, syndiotactic polypropylene has been known for a long period of time. However, the syndiotactic polypropylene prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound was poor in syndiotacticity and had elastomer-like characteristics. Therefore, it was scarcely considered to have characteristic properties as syndiotactic polypropylene.
On the contrary, syndiotactic polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 has been discovered for the first time by J. A. Ewen et al. which can be obtained by the use of a catalyst comprising a transition metal compound having an asymmetric ligand and an aluminoxane (J. Am. Chem. Soc., Vol. 110, p. 6255-6256, 1988). This syndiotactic polypropylene having a good steric regularity is excellent in balance between stiffness and impact resistance and therefore it is also sufficiently utilizable in the applications of the conventional isotactic polypropylenes. However, the above-mentioned catalyst has the problem that it can only provide polymers having a small molecular weight and a narrow molecular weight distribution and the other problem that it gives a poor moldability.
In order to solve these problems, it is useful to expand the molecular weight distribution, and this expansion of the molecular weight can be achieved to some extent by the use of a mixture of catalysts comprising different transition metals. However, in this case, there is the problem that the activity of the catalysts is poor.
Furthermore, when isotactic polypropylene is irradiated with radiation, the crosslinked and branched polymer can be obtained which is excellent in moldability (EP 190889 and EP 351866). When isotactic polypropylene is irradiated with radiation, however, its molecular weight decreases rapidly. Thus, it has not been considered that the molecular weight increases by a small dose, e.g., several Mrad of radiation [e.g., Journal of Applied Polymer Science, Vol. 11, p. 705 (1967)].